Acceleration sensors utilizing physical quantity change such as piezo-resistance effect or capacitance change have been developed and commercialized. Although these acceleration sensors can be widely used in various fields, there has recently been a demand for a smaller acceleration sensor capable of highly sensitively detecting acceleration in multi-axial directions at the same time.
Silicon single crystals have features: they can be an ideal elastic body because of their extremely few lattice defects, they can utilize semiconductor process technology without modification, and the like. Thus, particular attention is paid to semiconductor acceleration sensors of piezo-resistance effect type, in which thin flexible arms are formed on a silicon single crystal substrate and outputs are provided by converting stresses applied to the thin flexible arms into electric signals by a strain gauge such as % a piezo-resistor.
There are used three axis acceleration sensors having beam-structural flexible arms that are made of thin portions of a silicon single crystal substrate, connect a mass portion in the center made of a thick portion of the silicon single crystal substrate to a peripheral frame, and have a plurality of piezo-resistors formed in each axis direction on a flexible arm. In order to sensitively detect small acceleration, flexible arms are made longer and thinner, and the mass portion that serves as a pendulum is made heavier. The fact that small acceleration can be detected has caused flexible arms to be broken because great impact makes the amplitude of a mass portion too large. Thus, in order not to break flexible arms having had a large impact, regulation plates are provided above and below an acceleration sensor to regulate the amplitude of a mass portion by the regulation plates.
Patent Document 1 and Patent Document 2 show that in order to control the gap between a regulation plate and a mass portion of an acceleration sensor chip to be a predetermined value, microballs having substantially the same diameter as the gap are mixed with adhesive, and the adhesive mixed with the microballs is used to bond the regulation plate to the acceleration sensor chip. Since the gap between the regulation plate and the acceleration sensor chip can be determined by the diameter of a microball, the gap can be kept at the predetermined value. In this way, by using adhesive containing microballs, the gap between a regulation plate and an acceleration sensor chip can be controlled.
Piezo-resistors formed adjacent to a mass portion and adjacent to a support frame, respectively, on a flexible arm constitute a bridge circuit to measure each axial component of acceleration applied from the outside. The output voltage of the bridge circuit is as small as several mV to several 10 mV. Thus, a circuit for amplifying output voltage is required in order to address a wide range of application fields. Moreover, as for the output of a piezo-resistor, a temperature sensor for the piezo-resistor is needed to be located adjacent to an acceleration sensor to calibrate or compensate the output of the acceleration sensor with the output of the temperature sensor.
Thus, for example, Patent Document 3 proposes that an IC circuit including a bridge circuit, an amplifier circuit, and a temperature compensation circuit be incorporated in an upper regulation plate.
When an upper regulation plate having an IC circuit is used, heat generated by the IC circuit increases the temperature of the upper regulation plate. Since the upper regulation plate having the IC circuit is made of silicon, and the thermal conductivity of silicon is 168 w/m·K which is about 7000 times as large as that of air, 0.0241 w/m·K, the overall upper regulation plate is warmed by the heat generated by the IC circuit.
Piezo-resistors provided for an acceleration sensor chip are disposed on the top surfaces of flexible arms, that is, at locations opposite to the bottom surface of an upper regulation plate, at a small gap of several μm to several 10 μm from the bottom surface of the upper regulation plate, and hence when the temperature of the upper regulation plate increases, the temperature of the piezo-resistors is increased caused by radiant heat from the bottom surface of the upper regulation plate. The temperature of a piezo-resistor at a location corresponding to the center of the upper regulation plate, that is, adjacent to the mass portion, becomes lower or higher than the temperature of a piezo-resistor adjacent to the support frame. Since movement of air adjacent to the periphery of the acceleration sensor chip is larger than that of air at a center portion of the acceleration sensor chip, the temperature of a piezo-resistor formed adjacent to the support frame tends to vary from the temperature of a piezo-resistor formed adjacent to the mass portion. Since the piezo-resistors adjacent to the support frame and the piezo-resistors adjacent to the mass portion on the flexible arm are incorporated into the bridge circuit to measure each axial component of acceleration, if there is a temperature difference between the piezo-resistors adjacent to the support frame and the piezo-resistors adjacent to the mass portion, the temperature causes a resistance difference between them, and hence even when no acceleration is applied, an output voltage is measured. Specifically, there has been a problem that offset voltage is large.
Patent Document 1: Japanese Patent Laid-Open No. 4-274005
Patent Document 2: Japanese Patent Laid-Open No. 8-233851
Patent Document 3: Japanese Patent Laid-Open No. 6-242141